1. Field of the Invention
The present invention relates broadly to telecommunications systems and methods. More particularly, the present invention relates to a handshake for an xDSL (Digital Subscriber Line type) modem.
2. State of the Art
Digital subscriber line (DSL) systems are a new and fast-growing data transmission service which provide significantly higher data rates than conventional V.34 and V.90 type modems. The abbreviation xe2x80x9cxDSLxe2x80x9d is an integrated designation for different DSL services including ADSL (asymmetric DSL), SDSL (symmetric DSL), RADSL (rate-adaptive DSL), HDSL (high speed DSL), and VDSL (very high speed DSL), UDSL (universal DSL), and their modifications such as ADSL-LITE (also known as G.lite). The xDSL services typically provide data rates of several Mbits/s downstream and several hundred Kbits/s upstream, although SDSL provides the same upstream and downstream rates. All types of DSL are based on discrete multitone (DMT) technology although they have different parameters. See, J. Makris, xe2x80x9cDSL Servicesxe2x80x9d, Data Communications, April 1998, and ANSI T1.413-1995 xe2x80x9cNetwork and Customer Installation Interfacesxe2x80x94Asymmetrical Digital Subscriber Line (ADSL) Metallic Interfacexe2x80x9d.
According to the ITU-T telecommunications standards for the xDSL services, at modem start-up a handshake procedure (called G.hs) is utilized. The requirements for G.hs are set forth in several documents such as xe2x80x9cProposal for G.hs Modulation Technique and Message Protocolxe2x80x9d, TTU-T Telecommunication Standardization Sector, C1-068 Chicago, USA 6-9 Apr. 1998, and xe2x80x9cHandshake procedures for Digital Subscriber Line (DSL) transceiversxe2x80x9d, ITU-T Draft G.994.1 (Feb. 3, 1999) which are both hereby incorporated by reference herein in their entireties. The main requirements of the handshake are: transmission of several tens of bytes during the handshake; signal compatibility with all types of DSL receivers; and interworking with the plain old telephone service (POTS), the integrated services digital network (ISDN), and time compression multiplexing ISDN (TCM-ISDN). Meeting these main requirements is not a trivial task because of considerable noise and cross-talk impairments, and lack of knowledge regarding the frequency characteristics of the channel, all of which is described in various papers such as: Matsushita Electric Industrial Co. Ltd, xe2x80x9cProposed Working Text for G.hs Based on V.8bisxe2x80x9d, ITU-Telecommunication Standardization Sector, NF-044, Nice, France, 11-14 May 1998; Matsushita Electric Industrial Co. Ltd., xe2x80x9cSpectrum Considerations for G.hsxe2x80x9d, TTU-Telecommunications Standardization Sector, NF-045, Nice, France 11-14 May 1998; Matsushita Electric Industrial Co., Ltd., xe2x80x9cCrosstalk Model Proposed Working Text for G.hs Testxe2x80x9d ITU-Telecommunications Standardization Sector, NF-046, Nice, France 11-14 May 1998; NEC, xe2x80x9cDesired Spectrum Range for G.hs under TCM-ISDNxe2x80x9d, ITU-Telecommunications Standardization Sector, NF-066, Nice, France 11-14 May 1998; and 3Com, xe2x80x9cProposed Spectrum and Tone Selection for G.hsxe2x80x9d, ITU-Telecommunications Standardization Sector, NF-068, Nice, France 11-14 May 1998.
More particularly, signal attenuation across lines carrying xDSL signals is a non-monotonic function of frequency, and may have several deep notches, while noise power spectral density (PSD) is also not a flat function of frequency. As a result, the signal to noise ratio (SNR) is a complex multiextremes function of frequency. Moreover, the SNR is subject to random and cyclic variations in time. For example, in the TCM-ISDN environment which includes the so-called xe2x80x9cping-pong modexe2x80x9d of up-and down-transmissions, far-end cross-talk (FEXT) and near-end cross-talk (NEXT) interleave at a frequency of 400 Hz. Since FEXT and NEXT processes have significantly different power spectral densities, significant NEXT noise is introduced every other 1.25 milliseconds.
As set forth above, several authors have made proposals regarding G.hs techniques. The core of these proposals has been two-tone transmission with different bit rates depending upon the noise environment. Frequency diversity is provided by bits duplication on nominal and backup carrier tones. Time diversity is provided by increasing the symbol interval (i.e., decreasing the symbol rate). These proposals have several disadvantages. First, both the nominal and backup tones may be located in notches or other frequency domain areas having a low SNR, thus rendering the handshake ineffective. Second, increasing the symbol interval may not be sufficient to account for bursty noise. For example, in the TCM-ISDN environment, the signal to noise ratio may be below an acceptable level every other 1.25 ms interval. Even if the initial symbol interval of 0.232 ms were to be increased by a factor of four to 0.928 ms as suggested by one of the authors in the art, the entire interval could be located within the 1.25 ms high noise window. In fact, even increasing the symbol interval by a factor of 8 would still only provide a final symbol interval of 1.885 ms which could be 67% covered by the low SNR area.
It is therefore an object of the invention to provide a handshake for an xDSL modem which meets proposed xDSL standards requirements.
It is another object of the invention to provide a handshake for an xDSL modem which has excellent frequency diversity and time diversity and provides excellent reliability.
It is a further object of the invention to provide an xDSL modem handshake which utilizes multitone signaling.
It is an additional object of the invention to provide an xDSL modem handshake which will interwork with existing telecommunications services.
Another object of the invention is to provide modems and methods for implementing the above-listed objects.
In accord with the objects of the invention, handshake information for xDSL services are transmitted utilizing a spread spectrum modulated system where a plurality (n) of carrier tones (n greater than 2) are summed and utilized as a spread spectrum carrier (SSC), and data is modulated onto the carrier (at all utilized frequencies). Preferably, phase shift keying (PSK) modulation (or a variation thereof such as BPSKxe2x80x94binary PSK, or DBPSKxe2x80x94differential binary PSK) is used as the modulating technique. When the spread spectrum carrier is modulated by handshake bits according to BPSK, the SSC is transmitted with sign xe2x80x9c+xe2x80x9d if the handshake bit is a +1 and with sign xe2x80x9cxe2x88x92xe2x80x9d if the handshake bit is a xe2x80x9cxe2x88x921xe2x80x9d. When using DPSK, the same modulation procedure is used for differentially encoded handshake bits.
According to one preferred aspect of the invention, the handshake symbol rate (SR) is set equal to 0.8A symbols/msec, where A is a positive integer. In order to improve reliability, symbols are preferably repeated at least four times. According to another preferred aspect of the invention, a preamble can be provided for timing recovery purposes. Further aspects of the invention include different receiver systems, including a quasicoherent receiver, an autocorrelation receiver, and a presently preferred incoherent receiver which utilizes coherent accumulation of FFT components for a DBPSK spread spectrum handshake signal.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.